A gateway to this other dimensional realm is accessed via these alter type structures scattered on various planets. This realm is home to these advanced aliens that look kinda-sorta like the devil and want people to worship them. There is an intelligent alien virus/mycoplasma that comes along with the humans and is deadly to the aliens (if it wants to be).
Can you give us an estimate of when you read it (and any other details you happen to remember)?
Possibly Demons at the Rainbow Bridge by Jack Chalker Goodreads
Thanks so much. That’s it!
Great!